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Shear rams ensure oil-well safety
A spring-loaded blade interface and secondary cutting edge make even the toughest cuts in emergency situations
By Kevin Russelburg, Contributing Editor -- Design News, February 26, 2001
Houston, TX —Advances in the oil and gas drilling industry have resulted in an increase in the number and type of cables and tubing that extend into the well bore. Sometimes these materials—which range from thin cabling to 1/2-inch wall pipe and everything in between—must be sheared so that the well bore can be sealed off in an emergency situation, such as a sudden increase in pressure. Make an incomplete cut, and the well cannot be sealed off completely, leading to a potentially catastrophic situation.
To shear loose cable and multiple strings of tubing—the most difficult cutting requirements—engineers at Cameron designed a special mechanism called the DSI shear ram. It differs from traditional shear ram designs in that it has no space between the upper and lower cutting blades that can lead to incomplete cuts. Chamfered edges help avoid interference.
The problem with traditional rams, which have a space between the blades, is the tendency of loose cable to fold and lodge between the overlapping shear blades.
To eliminate this problem, the patented DSI shear rams have a special, spring-loaded blade interface with a secondary cutting edge. The upper shear has two beams that extend out and function as cantilevers, while the beams for the blade interface go under the lower shear blade during closure. "This fit has been designed to squeeze or pinch the lower blade tight against the upper blade to ensure absolute shear of the thinnest of wire, cable, screen, or composite strands," explains Albert Kachich, senior product design engineer.
The patent-pending, secondary cutting edge presents two cutting edges that the lower shear crosses under. During closure, the first cutting edge flattens the cable and cuts most of the cable strands. As the remaining strands begin to stretch between the overlapping blades, the secondary cutting edge cuts shears them.
A hydraulic piston provides the speed and high force (530 lbf) required to cut even the hardest of materials. In only 8 seconds after detection of a pressure buildup, the shears make a complete cut—preventing any high-pressure bubbles of gas from reaching the surface of the well at a high rate of acceleration.
Hardness of the blades was also an issue, but government regulations require that a soft steel be used for the blades. To get around this problem, engineers inlaid the cutting edge with a tool steel with the requisite hardness.
The shear rams have implications beyond safety: They are compatible with a new economic oil drilling system, which could possibly lead to lower energy bills for us all.
Additional Details
Albert KachichCameron,PO Box 1212 Houston, TX 77251; Tel: (713)939-2211; Fax: (713)939-2620; E-mail: kachich@camerondiv.com.
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